LDL (Calculated and Direct)

LDL-Cholesterol – Calculated and Direct

LDL–cholesterol plays a causal role in the development of atherosclerosis. Its measurement is therefore important for the diagnosis and treatment of hyperlipidaemia.
Most laboratories estimate LDL Cholesterol by the Friedewald equation:

LDLC = TC – HDLC -Trig/2.2

where Trig/2.2 approximates to VLDL Cholesterol.

This approximation assumes that total cholesterol is normally distributed across the three major lipoprotein classes. It is not valid at high triglyceride concentrations (>4.5mmol/L) or for non-fasting samples containing chylomicrons, when it under-estimates LDLC; or in Type III Hyperlipoproteinaemia, when it over-estimates LDLC.

Direct LDL measurements are indicated when the triglyceride concentration exceeds 4mmol/L. Beta-quantitation, based on ultracentrifugation, is the reference method, but automated methods evaluated against this procedure are now available.

Clinical indications:
LDLC concentration is a primary criterion which, together with other risk factors, is used in the treatment recommendations for hyperlipidaemia.

Reference Range:
LDL cholesterol (Target value) Less than 2.0mmol/L
(derived from patients with a >20% risk of CVD over 10 years)

Patient preparation:
Patients should follow their normal diet for 3 weeks prior to sampling. A fasting sample is required. Standardise posture to reduce effect of change in plasma volume – seat the patient for 5 minutes before sampling. Avoid venous stasis – apply tourniquet briefly before inserting the needle and release before drawing the sample.

Reference:
Friedewald WT, Levy RI, Fredrickson DS ;Estimation of the concentration of low-density lipoprotein cholesterol in plasma without the use of the preparative ultracentrifuge. Clin Chem 1972:18; 499-502